As the need for integrated circuits for semiconductor devices having higher performance and greater functionality increases, device feature geometries continue to decrease. As device geometries become smaller, the dielectric constant of an insulating material used between conducting paths becomes an increasingly important factor in device performance.
As device dimensions shrink to less than 0.25 μm, propagation delay, cross-talk noise and power dissipation due to resistance-capacitance (RC) coupling become significant due to increased wiring capacitance, especially interline capacitance between the metal lines on the same level. These factors all depend critically on the dielectric constant of the separating insulator or inter-layer dielectric (ILD).
The use of low dielectric constant (k) materials advantageously lowers power consumption, reduces cross talk, and shortens signal delay for closely spaced conductors through the reduction of both nodal and interconnect line capacitances. Dielectric materials that exhibit low dielectric constants are critical in the development path toward faster and more power efficient microelectronics.
Alkyl silanes, alkoxy silanes and polyhedral oligomeric silsesquioxanes (POSS) and other materials comprised mainly of Si, C, O and H (SiCOH) are being evaluated aggressively for obtaining low dielectric constant (k) thin-films as interlayer dielectrics in an integrated circuit by a PECVD approach. The resulting films formed when using these precursors give dense SiCOH containing films, having dielectric constants in the range of from about 2.4 to 3.2.
Introducing porosity to the low-dielectric constant SiCOH films may serve to further lower the dielectric constant to values below 2.5.
One particular class of precursors, cyclosiloxanes, (i.e. 2,4,6,8-tetramethylcyclotetrasiloxane (TMCTS)) is being considered as a source material for the deposition of low dielectric constant (k) thin-films used as interlayer dielectrics in an integrated circuit. Cyclosiloxanes provide a thin film having an open crystal structures or cage structure (e.g. Mantz et al., “Thermolysis of Polyhedral Oligomeric Silsesquioxane (POSS) Macromers and POSS-Siloxane Copolymers”, Chem. Mater., 1996, 8, 1250-1259). PECVD of thin films from such precursors results in open areas in the structure, which leads to low packing density and hence low k values.
Chemical vapor deposition (CVD) is the thin film deposition method of choice for large-scale fabrication of microelectronic device structures, and the semiconductor manufacturing industry has extensive expertise in its use.
The purification and reproducible delivery of cyclosiloxanes for CVD is extremely critical for full-scale commercialization of the thin-film process. At present the PECVD deposition process is suffering from irreproducible delivery due to polymerization of TMCTS within the delivery lines and process hardware. Questions related to the purification of TMCTS and elimination of the polymerization must be considered. The exact polymerization mechanism is presently not known. However, studies by the inventors of the instant invention indicate that catalytic polymerization of siloxanes occurs in the presence of water/moisture, acids and bases. Accordingly, there is a need in the art to reduce water content as well as other catalytic species from siloxanes, providing improved purity, stability and utility.
U.S. patent application Ser. No. 10/015,326, filed on Dec. 31, 2001 discloses a novel method for reducing water levels in cyclic siloxanes to levels as low as 2 ppm, by drying the material through physical absorption methods and/or azeotropic distillation. However, despite such efforts, a reliable method for the trace water analysis of cyclic siloxanes is not known.
Karl Fischer titration, a commonly used, quality control method for water analysis, fails to deliver accurate measurements of water for cyclosiloxane materials, as the Karl Fischer reagents are not compatible with many cyclosiloxanes. For example, Karl Fischer reagents based on the following redox reaction:SO2+I2+2H2O→H2SO4+2HI  (1)
However, in the case of cyclosiloxanes, the I2 reacts with the Si—H groups to give a false positive, according to the following mechanism (shown with tetramethylcyclotetrasiloxane):
and many cyclosiloxanes polymerize in the Karl Fischer reagents.
Fourier Transform Infra Red (FTIR) Spectroscopy, another commonly used analytical technique for measuring water concentrations in many chemicals has detection limits of greater than 40 ppm. As the acceptable water concentration for cyclic siloxanes may be on the order of less than 10 ppm, FTIR could not support such a measurement.
Gas Chromatography (GC) another commonly used analytical technique, reliably and accurately detects concentrations of volatile organics to levels of less than 5 ppm, by a vapor phase/substrate separation process in a capillary column. However, GC columns available for measurement of trace water fail to meet the same detection standards as for organic molecules, rendering the technique unacceptable for trace water measurements in cyclosiloxanes.
Accordingly, there is a need for an analytical method, which allows for the determination of trace water in cyclosiloxane materials.
Therefore, it is one objective of the present invention to develop an analytical technique for the measurement of trace water in cyclosiloxane materials.
It is a further objective of the present invention to develop an analytical technique for the accurate and precise measurement of trace water in cyclosiloxane materials having detection limits as low as 1 ppm.
It is a further objective of the present invention, to develop a method for the production of super dry cyclosiloxanes, which includes a means by which to efficiently and accurately measure the water content of the super dry cyclic siloxane material.